Solid-state graphene-based supercapacitor with high-density energy storage using ionic liquid gel electrolyte: electrochemical properties and performance in storing solar electricity

The electrochemical properties and high-density energy storage performance of graphene nano-platelet-based solid-state electrical double-layer supercapacitor device are reported. The graphene device is fabricated with electrolyte comprising of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) room temperature ionic liquid and LiClO4 dopant entrapped within polymer matrix formulated as a gel. The mesoporous graphene electrode was formed via dispersion in amorphous polyvinylidene (PVdF2) host over flexible graphite sheets with minimal graphene layer (< 5-layer) stacking. Exploiting the abundance of charge ion species in the ionic liquid gel electrolyte and pervasive accesses to the graphene platelets via voids, high double-layer specific capacitance of 214 Fg−1 was realized based on cyclic voltammetry data. Impedance studies show a low (0.79 Ω cm2) charge transfer resistance and a short Warburg range indicating highly diffusive ionic transport capability in the ionic liquid gel electrolyte. The Bode analysis showed high figure of merit for pulse power with 1145 ms response time and high-density (27 kWkg−1) pulsed power capability of the graphene supercapacitor. The charge–discharge data show graphene supercapacitor by availing high (~ 2 V) stable potential window in ionic liquid electrolyte gel greatly boosted the energy density to 33.3 Whkg−1 at power density 3 kWkg−1 with minimal decrement to 24.7 Whkg−1 at high ~ 3 Ag−1 discharge current density. By integration with solar cells, direct storage of light-generated electricity and discharge behavior of ionic liquid electrolyte graphene supercapacitor is reported.